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First thing I'm sorry if this question sounds too much beginner-ish. I don't major in electrical engineering so I may lack the basic knowledge.

I have been noticing that, while tuning my AM radio, nearly all stations have a "secondary frequency" that transmits the same audio as the advertised frequency. However, the quality of the audio is a little lower than the advertised frequency. For instance, the channel's advertised frequency is 675 kHz and I found the secondary frequency at 630 kHz. Another example is that main frequency at 549 kHz and secondary frequency at 729 kHz.

The pairs of main-secondary frequency do not seem to follow any patterns or rules. So my question is:

  • Is this on purpose (as a backup frequency or something else)?
  • If it's not on purpose, why does this happen?

More information: My radio is a basic consumer radio (Sony) with FM and AM (AM radio tuner is ranged from 531 kHz to 1602 kHz, with 9 kHz step between frequencies).

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    \$\begingroup\$ See if you can confirm these with another receiver of substantially different design, particularly one with a different IF. These don't sound like images with a traditional 455 KHz IF, but some modern designs use very low IF's... \$\endgroup\$ Sep 4, 2020 at 3:31
  • \$\begingroup\$ yeah, as @ChrisStratton said, this sounds like a receiver problem. Are you very close to the transmitter? \$\endgroup\$
    – P2000
    Sep 4, 2020 at 3:33
  • \$\begingroup\$ I don't have another radio, also I don't have any technical information on the radio I have. But I also recall that this happens on shortwave as well. While I was tuning the SDR on the internet (websdr.ewi.utwente.nl:8901), the same thing happened. So I guess it's not only me who experience this? \$\endgroup\$
    – MattCat15
    Sep 4, 2020 at 3:50
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    \$\begingroup\$ Hi MattCat15, Some Sony AM receivers (like the one in Sony Portable CD Player Boombox with AM/FM Radio & Cassette Tape Player) have a 45 kHz IF. Wonder if that is related to your problem of unwanted reception of a station 45 kHz down and another station 180 kHz up (45 * 4 =180). \$\endgroup\$
    – vu2nan
    Sep 4, 2020 at 10:12
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    \$\begingroup\$ @vu2nan You'd expect images spaced at twice the IF frequency, rather than at the IF, or at 4* the IF. This commonly used to map airport beacons transmitting a very slow Morse Code ID from about 300 kHz into the AM band. GLG was Glasgow airport. Those beacons have switched off now. \$\endgroup\$ Sep 4, 2020 at 12:25

2 Answers 2

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The secondary frequency most likely comes from a different transmitter transmitting the same service to a different part of the country. One is probably much stronger than another where you are : move to the other location and you'd get better reception from the other.

This page announces some of the changes in frequencies that are happening (UK). For example you might have found Radio Cornwall on 630 and 657kHz (but not 675kHz) : those channels are closing this year (maybe already have).

There is an incomplete list on Wikipedia : showing the above frequencies as Redruth and Bodmin (both towns in Cornwall) : oddly there are no entries for 675 kHz. It doesn't show any common broadcasts on 549 (Spirit Radio from Ireland?) and 729 kHz however.

They are all multiples of 9 kHz as this is the international standard channel spacing for (LW and MW) AM bands, except in N.America which uses 10kHz.

This also happens a lot on the Short Wave broadcast bands where you will hear e.g. BBC World Service on several bands at once, allowing you to choose whichever band has good propagation to your area at the moment, given the problems of fading and interference especially at night.

Here's a site showing who is on 630 kHz. Click on any other frequency there to see who's on that frequency. I don't know how complete it is.

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  • \$\begingroup\$ Yes, Brian, simultaneous transmission of the same programme content, at different frequencies and locations, is the most likely answer. \$\endgroup\$
    – vu2nan
    Sep 5, 2020 at 4:26
  • \$\begingroup\$ Thank you for your answer, Brian! I understand that some broadcasters have different frequencies for listeners to choose the best one that has the best audio quality, and they all should be announced. However in my case, the secondary frequencies are not announced. I searched for the 2 stations on their websites and they announce only one frequency for each. \$\endgroup\$
    – MattCat15
    Sep 5, 2020 at 7:31
  • \$\begingroup\$ Oh and I live in Asia, so the information for UK doesn't apply, sorry :) \$\endgroup\$
    – MattCat15
    Sep 5, 2020 at 7:35
  • \$\begingroup\$ Hi MattCat15, Here's another example - NHK Radio 1 is broadcast from Okinawa on 549 kHz AM, from Yamaguchi on 675 kHz AM and from Nagoya on 729 kHz AM. 630 kHz AM does not figure along with them! \$\endgroup\$
    – vu2nan
    Sep 5, 2020 at 10:07
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The effect you are noticing is not the radio station broadcasting on multiple frequencies i.e. not as a backup per your comment.

The reasons this could happen likely relate to the design and performance of your AM radio receiver. For example as mentioned if you are very close to the transmitting station the excessive signal power received within your radio will lead to intermodulation products (see https://en.wikipedia.org/wiki/Intermodulation for a description) giving the appearance of phantom images at other frequencies (if you search you may be able to find other frequencies beyond those you mentioned).

As suggested if you have access to another radio you might want to compare (another radio with a different design/manufacturer may have better intermodulation performance). If the alternative radio has a different IF it may give you a different frequency shift if the issue is still there. Also try using your radio at a different location with the same station as intermodulation products will be reduced at lower signal strength.

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    \$\begingroup\$ Thanks for your answer. However as far as I've researched, there are no transmitters nearby my place. \$\endgroup\$
    – MattCat15
    Sep 4, 2020 at 4:30
  • \$\begingroup\$ some radios have a switch to change the IF. \$\endgroup\$
    – Jasen
    Sep 4, 2020 at 4:33
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    \$\begingroup\$ The reported duplicate frequencies are 45kHz and 180kHz apart, which seem to be harmonics, perhaps 3*15kHz and 12*15kHz from a distorting oscillator or mixer. Either these harmonics are high or the filters after mixing to/from IF are deficient. \$\endgroup\$
    – P2000
    Sep 4, 2020 at 5:22
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    \$\begingroup\$ You might find this series of pages useful electronics-notes.com/articles/ham_radio/voice-modes/… If you want to go deeper consider rsgbshop.org/acatalog/Online_Catalogue_Training_19.html (Kindle) or somesuch \$\endgroup\$
    – mhaselup
    Sep 4, 2020 at 5:37
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    \$\begingroup\$ @MattCat15, mhaselup posted some links above. The topic of harmonics, distortion and intermodulation products is not "for dummies", but certainly worth learning as a topic on its own. Youtube covers it well: google.com/… \$\endgroup\$
    – P2000
    Sep 4, 2020 at 5:47

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